1. Field of the Invention
The present invention relates to a torque sensor, more particularly, to a torque sensor for detecting an engine torque in a mechanism for transmitting a revolution power of an electrically operated power steering apparatus.
2. Description of the Related Art
Conventionally, as a torque sensor of this kind, a torque sensor comprising an elastic member (torsion bar) for coaxially connecting two shafts (an input shaft and an output shaft) constituting a shaft member, a hard magnetic member (magnet) installed at one of two ends of the elastic member, a pair of tooth soft magnetic members (yokes) installed at another one of the two end portions, and a magnetometric sensor for detecting a magnetic flux density generated between the tooth soft magnetic members has been proposed, for example, by Japanese Patent Laid-Open No. 2003-149062 (JP-A-2003-149062) and published Japanese translation of PCT International publication No. 2004-519672 (JP-T-2004-519672).
In such torque sensor, a torque of the shaft member is detected as follows. When torsion is occurred in the elastic member, a magnetic pole position of the hard magnetic member and a tooth position of the soft magnetic member are changed relatively in a circumferential direction, so that a magnitude of the magnetic flux induced by the soft magnetic member is varied in accordance with the relative position of the magnetic pole position of the hard magnetic member and the tooth position of the soft magnetic member. The torque of the shaft member is detected by measuring the variation magnitude of the magnetic flux.
Now, in the torque sensor, a size of the magnet is limited by limitation in a total size of the sensor. In addition, the magnetic pole amplitude of the magnet becomes small, since a magnet having a large number of magnetic poles is used to improve torque detection accuracy for the torsion of the elastic member.
Therefore, in the conventional torque sensors disclosed by JP-A-2003-149062 and JP-T-2004-519672, a high strictness is required in dimensional accuracy between the magnet and the soft magnetic members. When variation exists in dimensions between the magnet and the soft magnetic members, a detection error is easily occurred, so that reliability in quality is decreased. Such a phenomenon is remarkable when a diameter of the magnet and the magnetic pole amplitude become small. As a result, there is a problem in that processing and assembling accuracy of the magnet and soft magnetic members becomes high, so that processing and assembling cost is increased as well as much time for processing and assembling is spent.
In addition, in the conventional torque sensors shown in JP-A-2003-149062 and JP-T-2004-519672, there is a problem in that the magnetic pole variation in a circumferential direction is generated, so that assembling of a magnetic yoke becomes difficult, as a result, an excellent assembling workability of the whole sensor cannot be obtained.